The volumes of the intracranial space and the craniospinal system as a whole change during the cardiac cycle. These volume changes are caused by the pulsatile arterial inflow to the cranium, venous outflow from the cranium, and cerebrospinal fluid (CSF) flow that oscillates back and forth between the cranium and the spinal canal. The volume changes can be measured accurately and reproducibly using a dynamic, motion-sensitive MRI technique [1]. It appears intuitive that the volume change of the entire craniospinal system (CSVC) should be greater than the intracranial volume change (ICVC). However, since they exhibit varying temporal information, CSVC can be smaller than ICVC. In the present study, these volume changes were measured in healthy humans and trauma cases. In the trauma cases, it was found that CSVC was smaller than ICVC. The cause was found to be increased pulsatility in the venous flow channels. It is suspected that the resulting relationship between ICVC and CSVC is related to the incidence of trauma, and perhaps CSVC being smaller than ICVC could serve as an indicator. The craniospinal system consists of two subcompartments, the intracranial space (skull) and the spinal canal. Cerebrospinal fluid (CSF) oscillates back and forth between these two subcompartments. The system can be modeled with arterial inflow and venous outflow. During systole, arterial blood rushes into the cranium, forcing the CSF out into the spinal canal. As the blood drains through venous channels during diastole, the CSF refills the cranium. Since this is a closed system, the entire craniospinal system volume change (CSVC) can be found by subtracting the venous (V) outflow from the arterial (A) inflow and examining the peak to peak change in the integral of A-V over one cardiac cycle. If the CSF flow is also subtracted from the A-V flow, the intracranial volume change (ICVC) can be obtained similarly. This method is used to measure the intracranial compliance and pressure from the ratio of volume and pressure changes that occur during the cardiac cycle [2]. II. METHODArterial, venous, and CSF flows were measured using a dynamic, motion-sensitive MRI technique [1]. This technique results in cross-sectional images of velocity. This means that the pixel values are proportional to the velocity of the particles. Gray areas are static; black represents velocity in one direction, while white represents velocity in the other. In the sample image (Fig. 2), the large white spot is a jugular vein carrying blood away from the cranium while the large black spots are carotid and vertebral arteries carrying blood into the cranium. The flow rates can be obtained from the images of velocity by integrating velocity values over the lumen area. This sample image is actually 1 of 32 images that are taken per cardiac cycle. Flow rates derived from each image are plotted together to create the flow waveforms in Fig. 3 that illustrate changing flow during one heart beat. Regular biases in the MRI velocity measurements that result from basel...